The present invention relates to narrow-band, reflective, multilayer, interference filters for blocking (reflecting) passage of laser or near-monochromatic radiation having a wavelength in the visible portion of the electromagnetic spectrum (visible wavelength), while transmitting other visible wavelengths. The invention is related in particular to such a filter wherein layers thereof are arranged such that the transmission characteristic of the filter is color neutral.
Narrow-band, reflective, multilayer, interference filters are extensively used in many types of laser-based direct-vision optical systems for protecting the eyes of a user from damage by stray laser radiation. One particular type of laser-based system in which such filters are employed is a laser surgery system which delivers laser radiation to a surgical site on a patient for tissue ablation, incision, cautery, or the like, while permitting a surgeon operating the system to observe the site and its surroundings during the operation.
In order to permit maximum transmission through such a filter it must be designed to reflect over as narrow a bandwidth as possible. Because of this, the filter typically has more individual layers and is of much greater thickness than a multilayer laser-cavity reflector which has no other requirements than an ability to reflect a particular laser wavelength to a desired degree.
A particular problem with prior art visible-wavelength laser blocking filters is that their reflection bandwidth, while significantly narrower than a conventional highly reflecting multilayer laser reflector, still blocks visible light at longer and shorter wavelengths than the laser radiation wavelength. Because of this, light which is transmitted through such prior-art filters has a pronounced color. For example, a filter that blocks (green) laser radiation having a wavelength about 530 nanometers (nm) will have a pronounced magenta color. For a surgeon using a laser-surgery system including such a filter, a pronounced transmission color can provide difficulty in properly assessing conditions in a surgical site being treated. Additionally, prior-art reflective-laser blocking filters often have a relatively-low photopic transmission, for example, less than about fifty percent, as a result of too great a reflection bandwidth. This can further complicate assessment of treatment conditions.